FIELD OF THE INVENTION
This invention relates to combustion with air in an aircraft traveling at a speed greater than the speed of sound, and more particularly to an apparatus to provide fueling for the combustor in this aircraft.
Efficient propulsion of aircraft must use engines that capture air in the atmosphere, mix this air with fuel, and bum the resulting mixture for thrust to move the aircraft forward. Subsonic combustors are not practical as supersonic (faster than the speed of sound) or hypersonic (much faster than the speed of sound) combustors because of the shock losses that result from having to slow the incoming air stream in order to complete combustion. Scramjets, or supersonic combustion ramjets, however, rely on increasing combustion pressure, and thus combustion efficiency, by slowing the incoming air to lower supersonic speeds. Current scramjet designs all use some form of supersonic combustion, and they all must solve the fundamental problems of this type of combustion, which are: 1) a low residence time in the combustor for fuel distribution and mixing, 2) the large aerodynamic losses caused by structures protruding into the flow, and 3) the low fraction of combustion energy relative to exhaust kinetic energy. The most difficult problem of supersonic combustion is to perform both mixing and full reaction during the very short residence time of the fuel in the combustor.
2. Description of the Related Art
It is one object of the present invention to decrease the time which is taken to mix all of the fuel and air on a molecular length scale so that complete combustion can take place.
Heretofore, using liquid injection of fuel it has been difficult to achieve vaporization of the liquid droplets, followed by gas mixing of this vapor with the surrounding air so that complete, molecular-scale mixing and combustion can take place inside the combustor. If the fuel droplets are small enough to vaporize quickly, they are carried along with the flow and because there is little relative velocity between the vaporizing droplet and the surrounding flow, there is no driving force for mixing the air with the vapor fuel except by molecular diffusion, which is very slow compared with fluid dynamic mixing. If the fuel droplets are large and a relative velocity can be maintained with respect to the surrounding air to promote mixing, a large amount of heat is required to vaporize the droplets and fuel vapor is formed at a relatively slow rate compared with the same mass of fuel dispersed in smaller droplets. Thus for either large or small liquid droplets of fuel the final mixing of fuel and air on a molecular scale necessary for combustion is a slow process compared with the time scales appropriate to flow through a supersonic combustor.
It is another object of the present invention to provide a method by which the fuel can be spread across the full extent of the combustor by injecting relatively large particles of fuel at high velocity through a wall that forms a side of the combustor or combustor inlet without protrusions from this wall.
Heretofore, using liquid injection of fuel it has been difficult to penetrate the supersonic airstream in the combustor to spread the fuel across the full extent of the combustor. In the presence of the high shear forces of supersonic flows, liquid fuel jets rapidly break up into tiny droplets that are then carried downstream with the flow before travelling very far across the flow duct. This problem has been overcome in the past by adding protrusions into the flow that are distributed over the inlet area from which fuel has been injected and can then completely fill the airstream. These protrusions cause a significant decrease in overall combustion efficiency because of the fluid mechanical losses that result from the shock waves that are caused by the protrusions.
Hydrogen fuel can be stored on the aircraft with a higher density than liquid hydrogen and yet be pumped with standard pumps as a stable slurry. Past art has utilized solid hydrogen slush extensively, but not commercially, for this purpose. Heretofore solid hydrogen has not been used as a fuel because of added cooling costs and because there has been no practical means of efficiently transporting it to and into the combustor. Solid hydrogen is preferable to liquid hydrogen because the same amount of material can be carried in the aircraft in a significantly smaller volume, decreasing the necessary size of the aircraft and decreasing the structure and structural weight needed to contain the fuel. Storing hydrogen as a solid is also a means for stabilizing energetic materials which would otherwise diffuse and react in a liquid fuel.
It is another object of the present invention to provide a separate motivation and means for using solid hydrogen for supersonic combustion with the understanding that solid hydrogen provides a method for freezing highly energetic materials in a stable state so that these energetic materials can be used to increase the energy of the fuel when mixing into the solid hydrogen. Normally solid or liquid hydrogen is gasified to absorb high-speed skin friction heating, rather than being injected into the combustor as a liquid or solid.